Memory cell that employs a selectively grown reversible resistance-switching element and methods of forming the same

ABSTRACT

A method of forming a memory cell is provided that includes forming a steering element above a substrate, forming a material layer on the substrate, patterning and etching the material layer, and oxidizing the patterned and etched material layer to form a reversible resistance-switching material. Numerous other aspects are provided.

REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.13/464,115, filed May 4, 2012, now U.S. Pat. No. 8,507,315, which is acontinuation of U.S. patent application Ser. No. 12/915,290, filed Oct.29, 2010, now U.S. Pat. No. 8,173,486, which is a continuation of U.S.patent application Ser. No. 11/772,082, filed Jun. 29, 2007, now U.S.Pat. No. 7,824,956.

This application is related to the following patent applications, eachof which is hereby incorporated by reference herein in its entirety forall purposes:

U.S. patent application Ser. No. 11/772,081, filed Jun. 29, 2007, nowabandoned, and titled “Method To Form A Rewriteable Memory CellComprising A Diode And A Resistivity-Switching Grown Oxide.”

U.S. patent application Ser. No. 11/772,090, filed Jun. 29, 2007, nowU.S. Pat. No. 7,846,785, and titled “Memory Cell That Employs ASelectively Deposited Reversible Resistance-Switching Element AndMethods Of Forming The Same.”

U.S. patent application Ser. No. 11/772,084, filed Jun. 29, 2007, nowU.S. Pat. No. 8,233,308, and titled “Memory Cell That Employs ASelectively Deposited Reversible Resistance-Switching Element AndMethods Of Forming The Same.”

BACKGROUND

The present invention relates to non-volatile memories and moreparticularly to a memory cell that employs a selectively grownreversible resistance-switching element and methods of forming the same.

Non-volatile memories formed from reversible resistance-switchingelements are known. For example, U.S. patent application Ser. No.11/125,939, filed May 9, 2005 and titled “Rewriteable Memory CellComprising A Diode And A Resistance-Switching Material” (hereinafter“the '939 Application”), which is hereby incorporated by referenceherein in its entirety for all purposes, describes a rewriteablenon-volatile memory cell that includes a diode coupled in series with areversible resistivity-switching material such as a metal oxide or metalnitride.

However, fabricating memory devices from rewriteableresistivity-switching materials is difficult; and improved methods offorming memory devices that employ reversible resistivity-switchingmaterials are desirable.

SUMMARY

In a first aspect of the invention, a method of forming a memory cell isprovided that includes forming a steering element above a substrate,forming a material layer on the substrate, patterning and etching thematerial layer, and oxidizing the patterned and etched material layer toform a reversible resistance-switching material.

In a second aspect of the invention, a method of forming a memory cellis provided that includes forming a first conductor above a substrate,forming a material layer on the substrate, patterning and etching thematerial layer, oxidizing the patterned and etched material layer toform a reversible resistance-switching material, forming a diode abovethe first conductor, and forming a second conductor above the diode andthe reversible resistance-switching element.

Other features and aspects of the present invention will become morefully apparent from the following detailed description, the appendedclaims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary memory cell providedin accordance with the present invention.

FIG. 2A is a simplified perspective view of a first embodiment of amemory cell provided in accordance with the present invention.

FIG. 2B is a simplified perspective view of a portion of a first memorylevel formed from a plurality of the memory cells of FIG. 2A.

FIG. 2C is a simplified perspective view of a portion of a firstexemplary three dimensional memory array provided in accordance with thepresent invention.

FIG. 2D is a simplified perspective view of a portion of a secondexemplary three dimensional memory array provided in accordance with thepresent invention.

FIG. 3 is a cross-sectional view of an exemplary embodiment of thememory cell of FIG. 2A.

FIGS. 4A-D illustrate cross sectional views of a portion of a substrateduring fabrication of a single memory level in accordance with thepresent invention.

FIG. 5 is a cross sectional view of an alternative memory cell providedin accordance with the present invention.

DETAILED DESCRIPTION

As stated above, fabricating memory devices from rewriteableresistivity-switching materials is difficult. For example, manyrewriteable resistivity-switching materials are difficult to etchchemically, increasing fabrication costs and complexity associated withtheir use in integrated circuits.

In accordance with the present invention, difficult-to-etch-chemicallyrewriteable resistivity-switching materials may be used within a memorycell without being etched. For example, in at least one embodiment, amemory cell is provided that includes a reversible resistivity-switchingmaterial formed using a selective growth process so that the reversibleresistivity-switching material may be used within the memory cellwithout being etched.

In one or more exemplary embodiments, a reversible resistance-switchingelement may be formed using titanium oxide as a reversibleresistivity-switching material. Titanium oxide films have been shown tobe suitable for use in memory cells, as described, for example, in the'939 Application, previously incorporated.

Titanium oxide films such as TiO, TiO₂, TiO_(x), TiO_(x)N_(y), etc., aredifficult to etch chemically. In at least one embodiment, through use ofa selective growth process, a titanium oxide layer may be used in areversible resistance-switching element of a memory cell without thetitanium oxide layer being etched. For example, a reversibleresistance-switching element may be formed by oxidizing atitanium-containing layer, such as titanium nitride, that is easier topattern and etch than titanium oxide. In this manner, only theunderlying titanium-containing layer (e.g., titanium nitride ortitanium) is patterned and/or etched prior to oxidation of thetitanium-containing layer and not the titanium oxide layer.

In some embodiments, titanium oxide may be selectively formed by rapidthermal oxidation of a titanium-containing layer in an oxygenenvironment such as O₂, ozone, a combination of the same, or using anyother suitable oxidizing species. In other embodiments, titanium oxidemay be formed by oxidizing a titanium-containing layer using oxygendiffusion in a chemical vapor deposition (“CVD”) chamber with an ozoneor other oxygen source, using gaseous or liquid ozone cleaning, or usingany other suitable oxidation process. In any case, the need for etchingof titanium oxide layers may be eliminated and memory cell fabricationsignificantly simplified.

Other materials may be selectively oxidized in accordance with thepresent invention to form reversible or one-time-programmableresistivity-switching materials for use in memory cells. For example, alayer of Ta, TaN, Nb, NbN, Al, AlN, Hf, HfN, V, VN, etc., may bedeposited on a substrate, patterned, etched and/or oxidized similarly toa titanium-containing layer so as to form a reversibleresistivity-switching material such as Ta₂O₅, Nb₂O₅, Al₂O₃, HfO₂, V₂O₅,etc.

Exemplary Inventive Memory Cell

FIG. 1 is a schematic illustration of an exemplary memory cell 100provided in accordance with the present invention. The memory cell 100includes a reversible resistance-switching element 102 coupled to asteering element 104.

The reversible resistance-switching element 102 includes a reversibleresistivity-switching material (not separately shown) having aresistance that may be reversibly switched between two or more states.For example, the reversible resistivity-switching material of theelement 102 may be in an initial, low-resistivity state upon fabricationthat is switchable to a high-resistivity state upon application of afirst voltage and/or current. Application of a second voltage and/orcurrent may return the reversible resistivity-switching material to alow-resistivity state. Alternatively, the reversibleresistance-switching element 102 may be in an initial, high-resistancestate upon fabrication that is reversibly switchable to a low-resistancestate upon application of the appropriate voltage(s) and/or current(s).When used in a memory cell, one resistance state may represent a binary“0,” while another resistance state may represent a binary “1,” althoughmore than two data/resistance states may be used. Numerous reversibleresistivity-switching materials and operation of memory cells employingreversible resistance-switching elements are described, for example, inthe '939 Application, previously incorporated.

In at least one embodiment of the invention, the reversibleresistance-switching element 102 is formed using a selective growthprocess. As will be described further below, use of a selective growthprocess allows a reversible resistivity-switching material to beprovided within the reversible resistance-switching element 102 withoutthe reversible resistivity-switching material having to be etched.Fabrication of the reversible resistance-switching element 102 therebyis simplified.

The steering element 104 may include a thin film transistor, a diode, oranother suitable steering element that exhibits non-ohmic conduction byselectively limiting the voltage across and/or the current flow throughthe reversible resistance-switching element 102. In this manner, thememory cell 100 may be used as part of a two or three dimensional memoryarray and data may be written to and/or read from the memory cell 100without affecting the state of other memory cells in the array.

Exemplary embodiments of the memory cell 100, the reversibleresistance-switching element 102 and the steering element 104 aredescribed below with reference to FIGS. 2A-5.

First Exemplary Embodiment of a Memory Cell

FIG. 2A is a simplified perspective view of a first embodiment of amemory cell 200 provided in accordance with the present invention. Withreference to FIG. 2A, the memory cell 200 includes a reversibleresistance-switching element 202 (shown in phantom) coupled in serieswith a diode 204 between a first conductor 206 and a second conductor208. In some embodiments, a barrier layer 209 such as titanium nitride,tantalum nitride, tungsten nitride, etc., may be provided between thereversible resistance-switching element 202 and the diode 204.

As will be described further below, the reversible resistance-switchingelement 202 is selectively formed so as to simplify fabrication of thememory cell 200. In at least one embodiment, the reversibleresistance-switching element 202 includes at least a portion of atitanium oxide layer formed by oxidizing a titanium-containing layersuch as titanium nitride. For example, a titanium nitride layer oranother similar form of titanium may be deposited above or below thediode 204, patterned and etched (e.g., such as with the first conductor206). The titanium nitride (or other) layer then may be oxidized to formtitanium oxide (e.g., using rapid thermal oxidation or another oxidationprocess).

In the embodiment of FIG. 2A, a titanium nitride or similar layer 210 isformed over, and is patterned and etched with, the first conductor 206.The titanium nitride or similar layer 210 then is oxidized to form atitanium oxide layer 212. A portion of the titanium oxide layer 212 thatvertically overlaps and/or aligns with the diode 204 may serve as thereversible resistance-switching element 202 between the diode 204 andthe first conductor 206 of the memory cell 200. In some embodiments,only a portion, such as one or more filaments, of the reversibleresistance-switching element 202 may switch and/or be switchable. Thetitanium oxide layer 212 may include, for example, TiO, TiO₂, TiO_(x),TiO_(x)N_(y) or the like. While the reversible resistance-switchingelement 202 is shown as being positioned below the diode 204 in FIG. 2A,it will be understood that in alternative embodiments, the reversibleresistance-switching element 202 may be positioned above the diode 204.Additional details for the reversible resistance-switching element 202are described below with reference to FIG. 3.

The diode 204 may include any suitable diode such as a verticalpolycrystalline p-n or p-i-n diode, whether upward pointing with ann-region above a p-region of the diode or downward pointing with ap-region above an n-region of the diode. Exemplary embodiments of thediode 204 are described below with reference to FIG. 3.

The first and/or second conductor 206, 208 may include any suitableconductive material such as tungsten, any appropriate metal, heavilydoped semiconductor material, a conductive silicide, a conductivesilicide-germanide, a conductive germanide, or the like. In theembodiment of FIG. 2A, the first and second conductors 206, 208 arerail-shaped and extend in different directions (e.g., substantiallyperpendicular to one another). Other conductor shapes and/orconfigurations may be used. In some embodiments, barrier layers,adhesion layers, antireflection coatings and/or the like (not shown) maybe used with the first and/or second conductors 206, 208 to improvedevice performance and/or aid in device fabrication.

As stated, other materials may be used to form the reversibleresistance-switching element 202. For example, materials such as Ta,TaN, Nb, NbN, Al, AlN, Hf, HfN, V, VN, etc., may be similarly depositedover (and/or patterned and etched with) the first conductor 206 and thenoxidized to form the layer 212, which includes the reversibleresistance-switching element 202.

FIG. 2B is a simplified perspective view of a portion of a first memorylevel 214 formed from a plurality of the memory cells 200 of FIG. 2A.For simplicity, the titanium-containing layer 210 and the titanium oxidelayer 212 are only shown on one of the bottom conductors 206. The memoryarray 214 is a “cross-point” array including a plurality of bit lines(second conductors 208) and word lines (first conductors 206) to whichmultiple memory cells are coupled (as shown). Other memory arrayconfigurations may be used, as may multiple levels of memory. Forexample, FIG. 2C is a simplified perspective view of a portion of amonolithic three dimensional array 216 that includes a first memorylevel 218 positioned below a second memory level 220. In the embodimentof FIG. 2C, each memory level 218, 220 includes a plurality of memorycells 200 in a cross-point array. It will be understood that one or moreadditional layers (e.g., an interlevel dielectric) may be presentbetween the first and second memory levels 218 and 220, but are notshown in FIG. 2C for simplicity. Other memory array configurations maybe used, as may additional levels of memory. In the embodiment of FIG.2C, all diodes may “point” in the same direction, such as upward ordownward depending on whether p-i-n diodes having a p-doped region onthe bottom or top of the diode are employed, simplifying diodefabrication.

In some embodiments, the memory levels may be formed, as described, forexample, in U.S. Pat. No. 6,952,030, “High-density three-dimensionalmemory cell” which is hereby incorporated by reference herein in itsentirety for all purposes. For instance, the upper conductors of a firstmemory level may be used as the lower conductors of a second memorylevel that is positioned above the first memory level as shown in FIG.2D. In such embodiments, the diodes on adjacent memory levels preferablypoint in opposite directions as described in U.S. patent applicationSer. No. 11/692,151, filed Mar. 27, 2007 and titled “Large Array OfUpward Pointing P-I-N Diodes Having Large And Uniform Current”(hereinafter “the '151 Application”), which is hereby incorporated byreference herein in its entirety for all purposes. For example, thediodes of the first memory level 218 may be upward pointing diodes asindicated by arrow A₁ (e.g., with p regions at the bottom of thediodes), while the diodes of the second memory level 220 may be downwardpointing diodes as indicated by arrow A₂ (e.g., with n regions at thebottom of the diodes), or vice versa.

A monolithic three dimensional memory array is one in which multiplememory levels are formed above a single substrate, such as a wafer, withno intervening substrates. The layers forming one memory level aredeposited or grown directly over the layers of an existing level orlevels. In contrast, stacked memories have been constructed by formingmemory levels on separate substrates and adhering the memory levels atopeach other, as in Leedy, U.S. Pat. No. 5,915,167, “Three dimensionalstructure memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree dimensional memory arrays.

FIG. 3 is a cross-sectional view of an exemplary embodiment of thememory cell 200 of FIG. 2A. With reference to FIG. 3, the memory cell200 includes the reversible resistance-switching element 202 (e.g., aportion of a layer of reversible resistivity-switching material, namelytitanium oxide layer 212 in this embodiment), the diode 204 and thefirst and second conductors 206, 208. The reversibleresistance-switching element 202 may be a portion of the titanium oxidelayer 212 that vertically overlies and/or overlaps with the diode 204.

In the embodiment of FIG. 3, the reversible resistance-switching element202 is formed by a selective growth process. For example, the titaniumoxide layer 212 may be selectively formed on the titanium-containinglayer 210 by oxidizing the titanium-containing layer 210. In thismanner, only the titanium-containing layer 210, and not the titaniumoxide layer 212, is etched, such as during the pattern and etch step(s)for the first conductor 206.

The titanium-containing layer 210 may be oxidized by any suitableprocess. For instance, the titanium-containing layer 210 may be oxidizedusing thermal oxidation in oxygen, ozone, a combination of the same oranother oxygen source (e.g., using rapid thermal oxidation).Alternatively or additionally, the titanium-containing layer 210 may beoxidized using oxygen diffusion in a CVD chamber with an ozone or otheroxygen source, using gaseous or liquid ozone cleaning, or using anyother suitable oxidation process to form titanium oxide. As stated,other reversible resistance-switching materials may be similarly formedby oxidizing Ta, TaN, Nb, NbN, Al, AlN, Hf, HfN, V, VN, etc.

In one exemplary embodiment, rapid thermal oxidation may be performed ata temperature of about 300° C. to about 800° C. for about one second toabout 5 minutes at an oxygen flow rate of about 2 sccm to about 40 sccm,depending on the desired oxide thickness and/or other properties. Otheroxidizing species, temperatures, times and/or flow rates may be used.

Oxidation by ozone diffusion in a CVD chamber may be performed at atemperature of about 300° C. to about 800° C., more preferably at atemperature of about 350° C. to about 450° C., for about 2 minutes toabout 4 hours, more preferably for about 15 to 25 minutes, at a suitableozone flow rate, such as between about 10 and 60 sccm, depending on thedesired oxide thickness and/or other properties. Other oxidizingspecies, temperatures, times and/or flow rates may be used.

In each of the above cases, only the titanium-containing layer 210 ispatterned and etched, and the need for etching of titanium oxide layersis eliminated. Memory cell fabrication is thereby significantlysimplified. Further, any desired thickness of titanium oxide may beformed. In some embodiments, a titanium oxide layer thickness of about500 angstroms or less, and more preferably about 300 angstroms or less,is employed for the reversible resistance-switching element 202(although other thickness ranges may be used).

As stated, the diode 204 may be a vertical p-n or p-i-n diode, which mayeither point upward or downward. In the embodiment of FIG. 2D in whichadjacent memory levels share conductors, adjacent memory levelspreferably have diodes that point in opposite directions such asdownward-pointing p-i-n diodes for a first memory level andupward-pointing p-i-n diodes for an adjacent, second memory level (orvice versa).

In some embodiments, the diode 204 may be formed from a polycrystallinesemiconductor material such as polysilicon, a polycrystallinesilicon-germanium alloy, polygermanium or any other suitable material.For example, the diode 204 may include a heavily doped n+ polysiliconregion 302, a lightly doped or an intrinsic (unintentionally doped)polysilicon region 304 above the n+ polysilicon region 302 and a heavilydoped, p+ polysilicon region 306 above the intrinsic region 304. In someembodiments, a thin (e.g., a few hundred angstroms or less) germaniumand/or silicon-germanium alloy layer (not shown), with about 10 at % ormore of germanium when using a silicon-germanium alloy layer, may beformed on the n+ polysilicon region 302 to prevent and/or reduce dopantmigration from the n+ polysilicon region 302 into the intrinsic region304, as described, for example, in U.S. patent application Ser. No.11/298,331, filed Dec. 9, 2005, and titled “Deposited SemiconductorStructure To Minimize N-Type Dopant Diffusion And Method Of Making”(hereinafter “the '331 Application”), which is hereby incorporated byreference herein in its entirety for all purposes. It will be understoodthat the locations of the n+ and p+ regions may be reversed.

In some embodiments, a barrier layer 308 such as titanium nitride,tantalum nitride, tungsten nitride, etc., may be formed between thetitanium oxide layer 212 and the n+ region 302 (e.g., to prevent and/orreduce migration of metal atoms into the polysilicon regions). Use ofsuch a metal barrier layer may form an unwanted rectifying contactbetween the barrier layer 308 and the titanium oxide layer 212.Accordingly, in some embodiments, a thin conductive layer (not shown),such as titanium, nickel, other conductive materials, etc., may beformed between the titanium oxide layer 212 and the barrier layer 308(e.g., for work function tuning, to reduce or prevent formation of arectifying contact).

When the diode 204 is formed from deposited silicon (e.g., amorphous orpolycrystalline), a silicide layer 310 may be formed on the diode 204 toplace the deposited silicon in a low resistivity state, as fabricated.Such a low resistivity state allows for easier programming of the memorycell 200 as a large voltage is not required to switch the depositedsilicon to a low resistivity state. For example, a silicide-formingmetal layer 312 such as titanium or cobalt, may be deposited on the p+polysilicon region 306. During a subsequent anneal step (describedbelow) employed to crystallize the deposited silicon that forms thediode 204, the silicide-forming metal layer 312 and the depositedsilicon of the diode 204 interact to form the silicide layer 310,consuming all or a portion of the silicide-forming metal layer 312.

As described in U.S. Pat. No. 7,176,064, “Memory Cell Comprising aSemiconductor Junction Diode Crystallized Adjacent to a Silicide,” whichis hereby incorporated by reference herein in its entirety,silicide-forming materials such as titanium and cobalt react withdeposited silicon during annealing to form a silicide layer. The latticespacings of titanium silicide and cobalt silicide are close to that ofsilicon, and it appears that such silicide layers may serve as“crystallization templates” or “seeds” for adjacent deposited silicon asthe deposited silicon crystallizes (e.g., the silicide layer 310enhances the crystalline structure of the silicon diode 204 duringannealing). Lower resistivity silicon thereby is provided. Similarresults may be achieved for silicon-germanium alloy and/or germaniumdiodes.

Following formation of the silicide-forming metal layer 312, the topconductor 208 is formed. In some embodiments, one or more barrier layersand/or adhesion layers 314 may be formed over the silicide-forming metallayer 312 prior to deposition of a conductive layer 315. The conductivelayer 315, barrier layer 314 and silicide-forming metal layer 312 may bepatterned and/or etched together to form the top conductor 208.

Following formation of the top conductor 208, the memory cell 200 may beannealed to crystallize the deposited semiconductor material of thediode 204 (and/or to form the silicide layer 310). In at least oneembodiment, the anneal may be performed for about 10 seconds to about 2minutes in nitrogen at a temperature of about 600 to 800° C., and morepreferably between about 650 and 750° C. Other annealing times,temperatures and/or environments may be used. As stated, the silicidelayer 310 may serve as a “crystallization template” or “seed” duringannealing for underlying deposited semiconductor material that forms thediode 204. Lower resistivity diode material thereby is provided.

An exemplary process for fabricating a memory cell in accordance withthe present invention is described below with reference to FIGS. 4A-D.

Exemplary Fabrication Process for a Memory Cell

FIGS. 4A-D illustrate cross sectional views of a portion of a substrate400 during fabrication of a first memory level in accordance with thepresent invention. As will be described below, the single memory levelincludes a plurality of memory cells that each include a reversibleresistance-switching element formed using a selective growth process.Additional memory levels may be fabricated above the first memory level(as described previously with reference to FIGS. 2C-2D).

With reference to FIG. 4A, the substrate 400 is shown as having alreadyundergone several processing steps. The substrate 400 may be anysuitable substrate such as a silicon, germanium, silicon-germanium,undoped, doped, bulk, silicon-on-insulator (“SOI”) or other substratewith or without additional circuitry. For example, the substrate 400 mayinclude one or more n-well or p-well regions (not shown).

Isolation layer 402 is formed above the substrate 400. In someembodiments, the isolation layer 402 may be a layer of silicon dioxide,silicon nitride, silicon oxynitride or any other suitable insulatinglayer. In other embodiments, the isolation layer 402 may be a shallowtrench isolation (“STI”) region formed by etching a trench in thesubstrate 400, depositing a dielectric such as silicon dioxide, siliconnitride or another dielectric over the substrate 400 to fill the trenchand planarizing the substrate 400 to re-expose a top surface 403 of thesubstrate 400. Note that in one or more embodiments, a silicon nitrideor similar protective layer (not shown) may be formed over activeregions (not shown) of the substrate 400 prior to isolation regionformation (e.g., to protect the active regions). As an alternative, alocalized oxidation of silicon (“LOCOS”) process or any other suitableprocess may be employed to define the isolation layer 402.

Following formation of the isolation layer 402, an adhesion layer 404 isformed over the isolation layer 402 (e.g., by physical vapor depositionor another method). For example, the adhesion layer 404 may be about 20to about 500 angstroms, and preferably about 100 angstroms, of titaniumnitride or another suitable adhesion layer such as tantalum nitride,tungsten nitride, combinations of one or more adhesion layers, or thelike. Other adhesion layer materials and/or thicknesses may be employed.In some embodiments, the adhesion layer 404 may be optional.

After formation of the adhesion layer 404, a conductive layer 406 isdeposited over the adhesion layer 404. The conductive layer 406 mayinclude any suitable conductive material such as tungsten or anotherappropriate metal, heavily doped semiconductor material, a conductivesilicide, a conductive silicide-germanide, a conductive germanide, orthe like deposited by any suitable method (e.g., chemical vapordeposition, physical vapor deposition, etc.). In at least oneembodiment, the conductive layer 406 may comprise about 200 to about2500 angstroms of tungsten. Other conductive layer materials and/orthicknesses may be used.

After formation of the conductive layer 406, a titanium-containing layer407, such as titanium nitride, is formed over the conductive layer 406(e.g., using physical vapor deposition or another method). In someembodiments, the titanium-containing layer 407 includes about 20 toabout 1200 angstroms of titanium nitride. Other titanium-containinglayer materials, such as titanium, a titanium alloy, TiSi₂, TiW, etc.,and/or thicknesses may be used.

Following formation of the titanium-containing layer 407, the adhesionlayer 404, the conductive layer 406 and the titanium-containing layer407 are patterned and etched. For example, the adhesion layer 404, theconductive layer 406 and the titanium-containing layer 407 may bepatterned and etched using conventional lithography techniques, with asoft or hard mask, and wet or dry etch processing. In at least oneembodiment, the adhesion layer 404, the conductive layer 406 and thetitanium-containing layer 407 are patterned and etched so as to formsubstantially parallel, substantially co-planar conductors 408 (as shownin FIG. 4A). Exemplary widths for the conductors 408 and/or spacingsbetween the conductors 408 range from about 200 to about 2500 angstroms,although other conductor widths and/or spacings may be used.

After the conductors 408 have been formed, a dielectric layer 410 isdeposited over the substrate 400 so as to fill the voids between theconductors 408. For example, approximately 3000-7000 angstroms ofsilicon dioxide may be deposited on the substrate 400 and planarizedusing chemical mechanical polishing or an etchback process to form aplanar surface 412. The planar surface 412 includes exposed, discreteregions 407 a-f of titanium-containing layer material 407 separated bydielectric material 410, as shown. The discrete titanium-containinglayer regions 407 a-f may be used to selectively form a titanium oxidereversible resistance-switching element for each memory cell beingformed above the substrate 400 (as described further below).

Other dielectric materials such as silicon nitride, silicon oxynitride,low K dielectrics, etc., and/or other dielectric layer thicknesses maybe used. Exemplary low K dielectrics include carbon doped oxides,silicon carbon layers, or the like.

If a reversible resistance-switching element is to be formed byoxidizing a material other than a titanium-containing material, thetitanium-containing layer 407 may be replaced with a layer of thematerial to be oxidized, such as Ta, TaN, Nb, NbN, Al, AlN, Hf, HfN, V,VN, etc.

Referring to FIG. 4B, following planarization, a reversibleresistance-switching element 413 a-f is formed over eachtitanium-containing layer region 407 a-f. For example, a titanium oxidelayer may be selectively formed over each titanium-containing layerregion 407 a-f by oxidizing the titanium-containing layer regions 407a-f. Some or all of each titanium-containing layer region 407 a-f may beconsumed during oxidation to create reversible resistance-switchingelements 413 a-f. As described previously, any suitable method may beemployed to oxidize the titanium-containing layer regions 407 a-f suchas rapid thermal oxidation in an oxygen environment such as O₂, ozone, acombination of the same, or using any other suitable oxidizing species.In other embodiments, a titanium-containing layer region may be oxidizedusing oxygen diffusion in a CVD chamber with an ozone or other oxygensource, using gaseous or liquid ozone cleaning, or using any othersuitable oxidation process to form titanium oxide.

With reference to FIG. 4C, after the reversible resistance-switchingelements 413 a-f have been formed, the diode structures of each memorycell are formed. An optional thin conductive layer (not shown), such asabout 10 to about 300 angstroms of titanium, nickel, etc., may be formedover the titanium oxide layer regions (e.g., for work function tuning).In some embodiments, a barrier layer 414, such as titanium nitride,tantalum nitride, tungsten nitride, etc., may also be formed over thetitanium oxide layer regions prior to diode formation (e.g., to preventand/or reduce migration of metal atoms into the polysilicon regions).The barrier layer 414 may be on top of, in addition to or in place ofthe thin conductive layer and may be about 20 to about 500 angstroms,and preferably about 100 angstroms, of titanium nitride or anothersuitable barrier layer such as tantalum nitride, tungsten nitride,combinations of one or more barrier layers, barrier layers incombination with other layers such as titanium/titanium nitride,tantalum/tantalum nitride or tungsten/tungsten nitride stacks, or thelike. Other barrier layer materials and/or thicknesses may be employed.

After deposition of the thin conductive layer (if used) and/or thebarrier layer 414, deposition of the semiconductor material used to formthe diode of each memory cell begins (e.g., diode 204 in FIGS. 2A-3).Each diode may be a vertical p-n or p-i-n diode as previously described.In some embodiments, each diode is formed from a polycrystallinesemiconductor material such as polysilicon, a polysilicon-germaniumalloy, germanium or any other suitable material. For convenience,formation of a polysilicon, downward-pointing diode is described herein.It will be understood that other materials and/or diode configurationsmay be used.

With reference to FIG. 4C, following formation of the barrier layer 414,a heavily doped n+ silicon layer 416 is deposited on the barrier layer414. In some embodiments, the n+ silicon layer 416 is in an amorphousstate as deposited. In other embodiments, the n+ silicon layer 416 is ina polycrystalline state as deposited. Chemical vapor deposition oranother suitable process may be employed to deposit the n+ silicon layer416. In at least one embodiment, the n+ silicon layer 416 may be formed,for example, from about 100 to about 1000 angstroms, preferably about100 angstroms, of phosphorus or arsenic doped silicon having a dopingconcentration of about 10²¹ cm⁻³. Other layer thicknesses, dopantsand/or doping concentrations may be used. The n+ silicon layer 416 maybe doped in situ, for example, by flowing a donor gas during deposition.Other doping methods may be used (e.g., implantation).

After deposition of the n+ silicon layer 416, a lightly doped, intrinsicand/or unintentionally doped silicon layer 418 is formed over the n+silicon layer 416. In some embodiments, the intrinsic silicon layer 418is in an amorphous state as deposited. In other embodiments, theintrinsic silicon layer 418 is in a polycrystalline state as deposited.Chemical vapor deposition or another suitable deposition method may beemployed to deposit the intrinsic silicon layer 418. In at least oneembodiment, the intrinsic silicon layer 418 may be about 500 to about4800 angstroms, preferably about 2500 angstroms, in thickness. Otherintrinsic layer thicknesses may be used.

A thin (e.g., a few hundred angstroms or less) germanium and/orsilicon-germanium alloy layer (not shown) may be formed on the n+silicon layer 416 prior to deposition of the intrinsic silicon layer 418to prevent and/or reduce dopant migration from the n+ silicon layer 416into the intrinsic silicon layer 418 (as described in the '331Application, previously incorporated).

Following formation of the n+ silicon layer 416 and the intrinsicsilicon layer 418, the n+ silicon layer 416, the intrinsic silicon layer418, the barrier layer 414 and/or any conductive layer (if used) arepatterned and etched so as to form silicon pillars 420 overlying theconductors 408 (as shown). Conventional lithography techniques, with asoft or hard mask, and wet or dry etch processing may be employed toform the silicon pillars 420.

After the silicon pillars 420 have been formed, a dielectric layer 422is deposited to fill the voids between the silicon pillars 420. Forexample, approximately 200-7000 angstroms of silicon dioxide may bedeposited and planarized using chemical mechanical polishing or anetchback process to form a planar surface 424. The planar surface 424includes exposed top surfaces of the silicon pillars 420 separated bydielectric material 422, as shown. Other dielectric materials such assilicon nitride, silicon oxynitride, low K dielectrics, etc., and/orother dielectric layer thicknesses may be used. Exemplary low Kdielectrics include carbon doped oxides, silicon carbon layers, or thelike.

After formation of the silicon pillars 420, a p+ silicon region 426 isformed within each silicon pillar 420, near the upper surface of thesilicon pillars 420. For example, a blanket p+ implant may be employedto implant boron a predetermined depth within the silicon pillars 420.Exemplary implantable molecular ions include BF₂, BF₃, B and the like.In some embodiments, an implant dose of about 1-5×10¹⁵ ions/cm² may beemployed. Other implant species and/or doses may be used. Further, insome embodiments, a diffusion process may be employed to dope the upperportion of the silicon pillars 420. In at least one embodiment, the p+silicon regions 426 have a depth of about 100-700 angstroms, althoughother p+ silicon region sizes may be used. (Note that if the diodes tobe formed are upward pointing p-n or p-i-n diodes, the upper portion ofthe silicon pillars 420 will be doped n-type). Each silicon pillar 420thereby includes a downward-pointing, p-i-n diode 428.

With reference to FIG. 4D, after completion of the p-i-n diodes 428, asilicide-forming metal layer 430 is deposited over the substrate 400.Exemplary silicide-forming metals include sputter or otherwise depositedtitanium or cobalt. In some embodiments, the silicide-forming metallayer 430 has a thickness of about 10 to about 200 angstroms, preferablyabout 20 to about 50 angstroms and more preferably about 20 angstroms.Other silicide-forming metal layer materials and/or thicknesses may beused. As will be described further below, annealing of the structurecauses metal from the silicide-forming metal layer 430 and silicon fromthe p+ silicon regions 426 to react to form a silicide region 432adjacent each p+ silicon region 426.

Following formation of the silicide-forming metal layer 430, a secondset of conductors 436 may be formed above the diodes 428 in a mannersimilar to the formation of the bottom set of conductors 408. In someembodiments, one or more barrier layers and/or adhesion layers 438 maybe placed over the silicide-forming metal layer 430 prior to depositionof a conductive layer 440 used to form the upper, second set ofconductors 436.

The conductive layer 440 may be formed from any suitable conductivematerial such as tungsten, another suitable metal, heavily dopedsemiconductor material, a conductive silicide, a conductivesilicide-germanide, a conductive germanide, or the like deposited by anysuitable method (e.g., chemical vapor deposition, physical vapordeposition, etc.). Other conductive layer materials may be used. Barrierlayers and/or adhesion layers 438 may include titanium nitride oranother suitable layer such as tantalum nitride, tungsten nitride,combinations of one or more layers, or any other suitable material(s).The deposited conductive layer 440, barrier and/or adhesion layer 438,and/or silicide-forming metal layer 430 may be patterned and/or etchedto form the second set of conductors 436. In at least one embodiment,the upper conductors 436 are substantially parallel, substantiallycoplanar conductors that extend in a different direction than the lowerconductors 408.

Following formation of the upper conductors 436, the structure may beannealed to crystallize the deposited semiconductor material of thediodes 428 (and/or to form the silicide regions 432). In at least oneembodiment, the anneal may be performed for about 10 seconds to about 2minutes in nitrogen at a temperature of about 600 to 800° C., and morepreferably between about 650 and 750° C. Other annealing times,temperatures and/or environments may be used. The silicide regions 432may serve as “crystallization templates” or “seeds” during annealing forunderlying deposited semiconductor material that forms the diodes 432(e.g., changing any amorphous semiconductor material to polycrystallinesemiconductor material and/or improving overall crystalline propertiesof the diodes 432). Lower resistivity diode material thereby isprovided.

Alternative Exemplary Memory Cell

FIG. 5 is a cross sectional view of an exemplary memory cell 500provided in accordance with the present invention. The memory cell 500includes a thin film transistor (“TFT”), such as a thin film, metaloxide semiconductor field effect transistor (“MOSFET”) 502 coupled to areversible resistance-switching element 504 formed above a substrate505. For example, the MOSFET 502 may be an n-channel or a p-channel thinfilm MOSFET formed on any suitable substrate. In the embodiment shown,an insulating region 506 such as silicon dioxide, silicon nitride,oxynitride, etc., is formed above the substrate 505 and a depositedsemiconductor region 507 such as deposited silicon, germanium,silicon-germanium, etc., is formed above the insulating region 506. Thethin film MOSFET 502 is formed within the deposited semiconductor region507 and is insulated from the substrate 505 by the insulating region506.

The MOSFET 502 includes source/drain regions 508, 510 and channel region512, as well as gate dielectric layer 514, gate electrode 516 andspacers 518 a-b. In at least one embodiment, the source/drain regions508, 510 may be doped p-type and the channel region 512 may be dopedn-type, while in other embodiments the source/drain regions 508, 510 maybe doped n-type and the channel region 512 may be doped p-type. Anyother MOSFET configuration or any suitable fabrication techniques may beemployed for the thin film MOSFET 502. In some embodiments, the MOSFET502 may be electrically isolated by isolation regions (not shown) formedin the substrate 506 (e.g., formed using an STI, LOCOS or other similarprocess). Alternatively, gate, source and/or drain regions of the MOSFET502 may be shared with other transistors (not shown) formed on thesubstrate 506.

The reversible resistance-switching element 504 includes a lowerconductor 520, a titanium-containing layer 521 formed over the lowerconductor 520, a titanium oxide layer 522 selectively grown over thetitanium-containing layer 521 and an upper conductor 524 formed over thereversible resistivity-switching material (titanium oxide layer 522).The upper and lower conductors 520, 524 may include any suitableconductive material such as tungsten, another metal, heavily dopedsemiconductor material, a conductive silicide, a conductivesilicide-germanide, a conductive germanide, or the like. In someembodiments, one or more barrier and/or adhesion layers (not shown) maybe provided between the upper and lower conductors 520, 524 and thereversible resistivity-switching material (titanium oxide layer 522).

In at least one embodiment, the reversible resistivity-switchingmaterial (titanium oxide layer 522) is formed using a selective growthprocess as previously described with reference to the embodiments ofFIGS. 1-4D. For example, the titanium oxide layer 522 may be selectivelyformed by rapid thermal oxidation of the titanium-containing layer 521in an oxygen environment such as O₂, ozone, a combination of the same,or using any other suitable oxidizing species. In other embodiments, thetitanium oxide layer 522 may be formed by oxidizing thetitanium-containing layer 521 using oxygen diffusion in a chemical vapordeposition (“CVD”) chamber with an ozone or other oxygen source, usinggaseous or liquid ozone cleaning, or using any other suitable oxidationprocess. In any case, the need for etching of titanium oxide layers maybe eliminated and memory cell fabrication significantly simplified.Other materials may be selectively oxidized in accordance with thepresent invention to form reversible resistivity-switching materials foruse in memory cell 500 (e.g., Ta, TaN, Nb, NbN, Al, AlN, Hf, HfN, V, VN,etc.).

As shown in FIG. 5, the reversible resistance-switching element 504 iscoupled to the source/drain region 510 of the MOSFET 502 by a firstconductive plug 526 and to a first metal level (M1) line 528 by a secondconductive plug 530 (which extend through a dielectric layer 532).Likewise, a third conductive plug 534 couples the source/drain region508 of the MOSFET 502 to an M1 line 536. The conductive plugs and/orlines may be formed from any suitable materials (without or withoutbarriers layers) such as tungsten, another metal, heavily dopedsemiconductor material, a conductive silicide, a conductivesilicide-germanide, a conductive germanide, or the like. Note that whenthe MOSFET 502 is an n-channel device, the region 508 serves as thedrain and the region 510 serves as the source for the MOSFET 502; andwhen the MOSFET 502 is an p-channel device, the region 508 serves as thesource and the region 510 serves as the drain for the MOSFET 502. Thedielectric layer 532 may include any suitable dielectric such as silicondioxide, silicon nitride, silicon oxynitride, low K dielectrics, etc.

In the memory cell 500, the thin film MOSFET 502 operates as a steeringelement in a manner similar to that of the diodes employed in the memorycells of FIGS. 2A-4D, selectively limiting the voltage applied acrossand/or the current flow through the reversible resistance-switchingelement 504.

In at least one embodiment, the reversible resistance-switching element504 includes a titanium oxide layer having a thickness of about 500angstroms or less, and more preferably a thickness of about 300angstroms or less. Other titanium oxide thicknesses may be employed.

The foregoing description discloses only exemplary embodiments of theinvention. Modifications of the above disclosed apparatus and methodswhich fall within the scope of the invention will be readily apparent tothose of ordinary skill in the art. For instance, although the presentinvention has been described primarily with regard to selectiveoxidation of titanium nitride, it will be understood that othermaterials may be selectively oxidized to form reversibleresistivity-switching materials for use in memory cells such as Ta, TaN,Nb, NbN, Al, AlN, Hf, HfN, V, VN, etc.

Accordingly, while the present invention has been disclosed inconnection with exemplary embodiments thereof, it should be understoodthat other embodiments may fall within the spirit and scope of theinvention, as defined by the following claims.

The invention claimed is:
 1. A method of forming a memory cell, themethod comprising: forming a diode above a substrate; and forming amaterial layer on the substrate; patterning and etching the materiallayer; and oxidizing the patterned and etched material layer to form areversible resistance-switching material.
 2. The method of claim 1,wherein the material layer comprises one or more of Ta, TaN, Nb, NbN,Al, AIN, Hf, HfN, V, and VN.
 3. The method of claim 1, wherein thereversible resistance-switching material comprises one or more of Ta₂O₅,Nb₂O₅, Al₂O₃, Hf0 ₂, and V₂O₅.
 4. The method of claim 1, wherein formingthe diode comprises forming a polycrystalline diode.
 5. The method ofclaim 1, wherein forming the diode comprises forming a verticalpolycrystalline diode.
 6. The method of claim 1, wherein forming thediode comprises forming a p-n diode or a p-i-n diode.
 7. The method ofclaim 1, wherein the reversible resistance-switching element has anoxide thickness of about 500 angstroms or less.
 8. The method of claim1, wherein the reversible resistance-switching element has an oxidethickness of about 300 angstroms or less.
 9. The method of claim 1,further comprising coupling the steering element and reversibleresistance-switching element in series.
 10. A memory cell formedaccording to the method of claim
 1. 11. A method of forming a memorycell, the method comprising: forming a first conductor above asubstrate; forming a material layer on the substrate; patterning andetching the material layer; and oxidizing the patterned and etchedmaterial layer to form a reversible resistance-switching material;forming a diode above the first conductor; and forming a secondconductor above the diode and the reversible resistance-switchingelement.
 12. The method of claim 11, wherein the material layercomprises one or more of Ta, TaN, Nb, NbN, Al, AIN, Hf, HfN, V, and VN.13. The method of claim 11, wherein the reversible resistance-switchingmaterial comprises one or more of Ta₂O₅, Nb₂O₅, Al₂ 0 ₃, HfO₂, and V₂O₅.14. The method of claim 11, wherein forming the diode comprises forminga vertical polycrystalline diode.
 15. The method of claim 14, furthercomprising forming a silicide, silicide-germanide or germanide region incontact with polycrystalline material of the vertical polycrystallinediode so that the polycrystalline material is in a low-resistivitystate.
 16. The method of claim 11, wherein the reversibleresistance-switching element has an oxide thickness of about 500angstroms or less.
 17. The method of claim 11, wherein the reversibleresistance-switching element has an oxide thickness of about 300angstroms or less.
 18. The method of claim 11, further comprisingcoupling the diode and the reversible resistance-switching element inseries.
 19. A memory cell formed according to the method of claim 11.20. A monolithic three dimensional memory array comprising: a firstmemory level formed above a substrate, the first memory level comprisinga plurality of memory cells, each memory cell formed according to themethod of claim 1; and at least a second memory level monolithicallyformed above the first memory level.
 21. A monolithic three dimensionalmemory array comprising: a first memory level formed above a substrate,the first memory level comprising a plurality of memory cells, eachmemory cell formed according to the method of claim 11; and at least asecond memory level monolithically formed above the first memory level.